Nice sentence! Here are clearer ways to say it and why.
Best simple version: In autumn, when it is rainy, the forest is full of mushrooms.
Other correct versions:
- In rainy autumns, the forest is full of mushrooms. (This means many autumns that are rainy.)
- In the rainy autumn, the forest is full of mushrooms. (This means a particular autumn that is rainy.)
Why these are better:
- We often say "in autumn" to mean the season.
- If you mean many autumns, say "autumns" (with an s).
- If you mean one special autumn, say "the rainy autumn."
- Saying "when it is rainy" is an easy way to explain the wet weather.
Try this aloud: "In autumn, when it is rainy, the forest is full of mushrooms."